The objective of the proposed experiments is to provide data in a model experimental system for evaluation of the theory of rectangularization of survivorship. This theory and the general alternative view make drastically different predictions about the age-structure of human populations in the next century. Specific objectives are: (i) To measure survivorship among the oldest survivors of 50,000 flies from inbred lines of Drosophila melanogaster. These data will allow accurate determination of the force of mortality at extreme ages, and a test for the existence of genotype-specific "caps" on longevity. (ii) To obtain survivorship data on the oldest of 50,000 flies derived from crosses between inbred lines. These data will provide further information about the existence of caps, independent of biases that arise as a result of complete homozygosity in inbred lines. (iii) To test a heterogeneous population of Drosophila for the presence of a critical prerequisite for the evolution of a rectangularized survivorship curve: genetic variation that promotes age-specific mortality. This will be investigated by an artificial selection experiment. (iv) To construct an archive of the data collected in the above experiments for the use of other biologists interested in survivorship and genetic variability of aging. (v) To analyze the data on the 50,000 files in experiments 1 and 2 using methods of life-table analysis and survival analysis to answer some specific questions of cross-cutting relevance to this program project.